Complete build options for Pthread API

[elfring]

Would you like to add the configuration script "AX_PTHREAD" to your build specification?

[dcsommer]

Did you run into an issue here? Feel free to send a PR if so. I'm taking the same approach as fbthrift and folly, so I'd lean towards consistency between these projects first.

[elfring]

The other build specifications contain similar update candidates, don't they?

[dcsommer]

@elfring yes, they do have similar update candidates. Long term we are looking at converting to cmake, so this enhancement is probably a wontfix. However, please do report any breakages you find!